The present invention relates to subterranean operations using slurries and, more particularly, to methods and compositions for reducing particle segregation in slurries.
Subterranean operations often utilize slurries of solid particulates. One such operation is hydraulic fracturing—a production stimulation wherein a formation is treated to increase its permeability by hydraulically fracturing the formation to create or enhance one or more cracks or “fractures.” In most cases, a hydraulic fracturing treatment involves pumping a proppant-free, viscous fluid (known as a pad fluid) into a subterranean formation faster than the fluid can escape into the formation so that the pressure in the formation rises and the formation breaks, creating an artificial fracture or enlarging a natural fracture. Then proppant particulates are generally added to the fluid to form a slurry that is pumped into the fracture to prevent the fracture from closing when the pumping pressure is released.
An example of a well completion operation using solid particulates is gravel packing. Gravel packing treatments are used, inter alia, to reduce the migration of unconsolidated formation particulates into the well bore. In gravel packing operations, particulates known in the art as gravel are carried to a well bore by a treatment fluid. That is, the particulates are slurried in a treatment fluid, which may be viscosified, and the treatment fluid is pumped into a well bore in which the gravel pack is to be placed. The treatment fluid leaks off into the subterranean zone and/or is returned to the surface while the particulates are left in the zone. The resultant gravel pack acts as a filter to separate formation sands from produced fluids while permitting the produced fluids to flow into the well bore. Typically, gravel pack operations involve placing a gravel pack screen in the well bore and packing the surrounding annulus between the screen and the well bore with gravel designed to prevent the passage of formation sands through the pack. The gravel pack screen is generally a type of filter assembly used to support and retain the gravel placed during the gravel pack operation. Such gravel packs may be used to stabilize the formation while causing minimal impairment to well productivity. The gravel, inter alia, acts to prevent formation sands from occluding the screen or migrating with the produced fluids, and the screen, inter alia, acts to prevent the gravel from entering the well bore.
In some situations the processes of hydraulic fracturing and gravel packing are combined into a single treatment to provide stimulated production and an annular gravel pack to reduce formation sand production. Such treatments are often referred to as “frac pack” operations. In some cases, the treatments are completed with a gravel pack screen assembly in place, and the hydraulic fracturing treatment being pumped through the annular space between the casing and screen. In such a situation, the hydraulic fracturing treatment usually ends in a screen out condition creating an annular gravel pack between the screen and casing. This allows both the hydraulic fracturing treatment and gravel pack to be placed in a single operation.
To prevent the subsequent flow-back of proppant, gravel, or other particulates with the produced fluids, a portion of the proppant introduced into the fractures may be coated with a tackifying agent that may facilitate the consolidation the proppant particles in the fracture. Typically, the coated proppant is deposited in the fracture after a large quantity of uncoated proppant has been deposited therein. The partially closed fractures apply pressure to the coated proppant particulates whereby the particulates are forced into contact with each other while the tackifying agent enhances the grain-to-grain contact between individual proppant particles. The action of the pressure and the proppant coating bring about the consolidation of the proppant particles into a permeable mass having compressive and tensile strength, serving to prevent unconsolidated proppant and formation sand from flowing out of the fractures with the produced fluids and damaging production equipment and/or the potential production of the well.
The use of such tackifying agents has proven particularly useful in hydrocarbon and water production. Tackifying agents, by their nature, remain sticky or retain adhesive character even after being placed in the formation. Thus, as formation fines attempt to flow through the pack with formation fluids, they can become entrained with the tackifying agent so as to not be produced with produced fluids. Tackifying agents may have some disadvantages, however. For example, traditional tackifying agents typically transform the carrier fluid to make it a hydrocarbon-based or non-aqueous carrier fluids, which may prove problematic in certain surface terrestrial, aquatic, or marine environments. Additionally, traditional agents often fail to provide the highly elastic proppant-to-proppant bond that is desirable in certain subterranean formations.
Furthermore, some fracturing, gravel packing, and frac-packing treatments are made more difficult due to problems with proppant placement. Often, it would be desirable to place proppant slurries containing a substantially uniform mixture of heterogeneous particles, such as different proppant materials or non-proppant solid particulates, to take advantage of properties offered by the mixture of materials. However, due to differences in, inter alia, settling rate, density, or size, these materials may segregate themselves during pumping and placement. This may result in a heterogeneous mixture, rather than the desired homogeneous mixture of materials, reducing the effectiveness of the proppant placement and possibly limiting the production potential of the well.